MPs have already backed the proposed law, authorising Prime Minister Theresa May to inform the EU of the UK’s intention to leave.

Opposition peers want to amend the bill at a later date to guarantee the rights of EU citizens in Britain and the role of Parliament in scrutinising the process.

As the government does not have a majority in the Lords, it is vulnerable to being outvoted if opposition peers – including Labour’s 202 and the 102 Lib Dems – join forces.

Mrs May has said she wants to invoke Article 50 of the 2009 Lisbon Treaty – the formal two-year mechanism by which a member state must leave the EU – by the end of March, and the government has warned the House of Lords not to frustrate the process.

Lord Heseltine writes: “The fightback starts here. My opponents will argue that the people have spoken, the [Brexit] mandate secured and the future cast. My experience stands against this argument”.

The former deputy prime minister, whose leadership challenge to Margaret Thatcher helped trigger her exit from Number 10 in 1990, campaigned for Remain in the run-up to the referendum.

He has been a supporter of the EU within the Conservative Party and backed the idea of the UK joining the single currency.